1. Field of the Invention
The present invention relates to novel diaphragms comprising asbestos fibers adapted for use in electrolytic cells, to the coupling of such diaphragms with a cathode component of such cells and to a process for producing such diaphragms and coupling such diaphragms with a cathode component.
This invention especially relates to improved diaphragms produced via a wet route, based on asbestos fibers, and adapted for use in chlorine/soda electrolysis cells.
2. Description of the Prior Art
Asbestos fibers have long been employed in this art as a conventional material for producing the diaphragms used in electrolytic cells. These diaphragms are fabricated by depositing asbestos fibers contained in an aqueous mash onto a cathode which is permeable to the electrolytes, the deposition operation being carried out under vacuum. Thus, French Patent No. 2,213,805 describes preparing microporous separators by depositing a layer of asbestos, said layer being consolidated by a fluoropolymer. The porosity of such a layer can be better controlled by adding a pore-forming agent according to the technique described in French Patent No. 2,229,739.
As is well known to this art, such preparation of microporous separators by depositing, under vacuum, an aqueous mash containing fibers and a binder presents a very great advantage, both from a technological standpoint, as well as from an economic standpoint. However, the quality of the separators thus produced is not fully satisfactory.
Indeed, the Faraday yield is insufficient, and this is reflected in a high energy consumption per ton of chlorine produced. The higher the sodium hydroxide concentration, the more the yield in question is lowered on an industrial level. Thus, it is of the greatest importance to be able to operate with concentrated sodium hydroxide, in order to reduce the energy cost of the evaporation which is subsequently required to concentrate the sodium hydroxide produced. It would, therefore, be desirable to provide an improved diaphragm based on asbestos fibers and capable of being produced by a wet route.
In published European Patent Application No. 132,425 cathode components have been described, composite material produced by the coupling of an elementary cathode including a highly porous metal surface such as a metal grid having a mesh opening ranging from 20 .mu.m to 5 mm and of a sheet containing fibers and a binder, the coupling and the sheet resulting from the programmed suction under vacuum of a suspension containing essentially electrically conductive fibers and a fluoropolymer, directly through said elementary cathode, followed by drying, and then melting the binder. Such composite materials that are adapted to themselves constitute the cathode of an electrolysis cell and may be coupled with a diaphragm, it being possible for the diaphragm to be manufactured directly by a wet route on the composite.
Various improvements have also been made, both to the composite materials themselves, as well as to the process for the manufacture thereof.
In published European Patent Application No. 214,066 materials are described, containing carbon fibers exhibiting a monodisperse length distribution, materials whose quality and properties are very appreciably improved, and this is reflected in a much more favorable performance/thickness relationship.
In published European Patent Application No. 296,076 electroactivated materials are described which contain an electrocatalytic agent uniformly distributed within their bulk mass, said agent being selected from among Raney metals and Raney alloys from which most of the easily removable metal(s) has (have) been removed.
The assemblage of proposed cathode components which ensure an appreciable distribution of the current is adapted for use in an electrolytic cell which will comprise a membrane or a diaphragm between the anode and cathode compartments. Additional technical details are described in the aforementioned European patent applications, hereby expressly incorporated by reference in respect of the construction of said cathode components.